I need to hook up 2 sinks ( 1 gal/min max each) on the same wall on the same floor. Do I need 2 vertical back vents @ 1 1/2" or will 1 do? If I vent up from sink 2 it is 12 feet to the main 3" roof vent. There are no fixtures above these. Check my drawing. Thanks, and congratulations on a great site.